Brake drums are subjected to relatively high pressures and high temperatures in service. Iron alloys are particularly suited to provide braking action in contact with the lining of brake shoes, but are relatively weak. There have been many proposals for strengthening brake drums, the most successful of which is described in Van Halteren, et al., U.S. Pat. No. 2,316,029, and involves centrifugally casting an iron alloy liner into a steel shell of the desired shape. Centrifugal casting, however, is relatively expensive and time consuming compared to static casting processes such as sand casting. In addition, centrifugal casting requires special equipment which places limitations on the shape of the steel shell in order to fit on the turntable of the centrifugal casting machinery. In addition, the heat and centrifugal forces involved in centrifugal casting may combine in some cases to cause the cast brake drum to go slightly out-of-round. Therefore, centrifugally cast liners must be made thicker than desired for purely functional reasons in order to provide sufficient machining material to correct the out-of-round condition.
Other attempts to develop composite brake drums having the high strength of steel but the metallurgical characteristics of cast iron on the inner friction surface are represented in Norton, U.S. Pat. No. 3,841,448 which discloses an encircling steel band and Bush, U.S. Pat. No. 4,858,731 which has an embedded steel wire framework.
The wire framework of Bush has proved of little assistance as the steel wires must be of small diameter so that the steel wires will heat and expand at nearly the same rate as the cast iron surrounding them. In practice, the stresses applied from the brake shoes acting on the drum are placed largely on a single strand of the reinforcing steel wire--until that wire breaks. Then the stresses are placed on an adjacent wire until it similarly fails, and this process is repeated until the entire reinforcing framework is broken. Similarly in Norton, the reinforcing band does not offer the reinforcing strength of a complete steel shell or an inward steel flange for attachment to a wheel or bonnet.